To persons skilled in the art of the transmission of electrical power at high voltages, it is known to provide the transmission lines with power circuit breakers. These are structures of substantial size, being on the order of 10 to 20 feet tall and having, in the vicinity of their tops, electrically insulated and supported contacts that may be rapidly opened whenever an overload or fault occurs on the transmission line being protected by such breaker. When the contacts of such a power circuit breaker are opened, an arc results, and it is naturally desirable that the arc be extinguished as quickly as possible in order to avoid damage to the circuit breaker. Moreover, with voltages as high as 750 kilovolts, the arc may be several inches long, or even as long as a few feet.
It has become customary in certain designs to provide the contact area of a power circuit breaker with a flow of sulfur hexafluoride. Sulfur hexafluoride (SF.sub.6) is a gas at room temperature and atmospheric pressure, and it is chemically rather inactive. It has a dielectric value substantially higher than that of air, so that an electric arc therein not only tends to be smaller, i.e., more filamentary, but also to decay and be extinguished substantially more rapidly.
However, an electric arc causes degradation of sulfur hexafluoride into chemical entities that are extremely reactive, such as positively or negatively charged fluorine atoms and the like. These chemical entities are capable of abstracting hydrogen from molecules having an O-H bond or other active hydrogens, to form hydrogen fluoride, which is extremely reactive to many insulating materials. The reactivity of arced sulfur hexafluoride is aggravated by the presence of moisture, and moisture cannot always be completely excluded from the vicinity of the contacts of a power circuit breaker.
In building power circuit breakers of the kind protected with sulfur hexafluoride, it has been customary to lead the SF.sub.6 gas from a compressor and high-pressure reservoir through a feed tube, wherein the SF.sub.6 gas is under pressure of about 250 pounds per square inch, to the vicinity of the contacts, where SF.sub.6 gas is maintained at a lower pressure such as 50 pounds per square inch. The feed tube may be visualized as a simple cylindrical tube, about 12 feet long, 3 inches in outside diameter, and 1/4 inch in wall thickness. It had been customary to make such feed tubes by coating a sheet of paper on one or both sides with resin, and rolling the paper to form the feed tube. Modern design provides for coating the resin on a steel mandrel, and curing the resin to form a feed tube liner. This is followed by wrapping resin coated glass fabric or filaments on the liner, to form the feed tube.
The development of a satisfactory feed tube liner for use in power circuit breakers of the kind using SF.sub.6 involves more than just finding a material that is chemically resistant to the reactive entities present in arcing SF.sub.6. A resinous material for this purpose must also be reasonably convenient to handle, must cure without sag drainage so as to provide straight, uniform liners, and it must also possess adequate arc and track resistance.
Sag resistant coatings for feed tube liners, useful in atmospheres of moist, pressurized sulfur hexafluoride insulating gas, found in many extra-high-voltage power circuit breakers, are known in the art, and taught by Luck et al. in U.S. Pat. No. 3,828,000. Such coatings, however, were made from filled, flexibilized, cycloaliphatic epoxy resin compositions utilizing asbestos filler as a critical thickening agent. However, the use of asbestos-containing materials has been severely curtailed in industry because of potential health hazards.
Polyester resins, vacuum pressure impregnated into the interstices of electrical coil wrappings have been quick gelled, to prevent resin drainage from the wrapping interior, by ultraviolet irradiation for a two minute period before final 125.degree. C. to 165.degree. C. cure, as taught by Gruenwald, in U.S. Pat. No. 3,937,855. However, as taught by Luck et al., in U.S. Pat. No. 4,102,851, polyesters are not generally chemically resistant to arced sulfur hexafluoride that might be found during overload of an extra-high-voltage power circuit breaker.
Additionally, Luck et al., in U.S. Pat. No. 4,102,851, teaches that bisphenol A epoxy resins, polyurethane resins, polyamide resins, polyphenylene oxide resins, silicone resins, phenolic resins, and many other materials, would not be useful in circuit breaker feed tube liners, or other coated breaker articles. It had always been thought that resin modification of the cycloaliphatic epoxy base resin used to make feed tube liners, would produce disastrous results in an arced SF.sub.6 environment. Luck et al. solved the asbestos problem, not by resin modification, but by substituting finely divided aluminum oxide filler for the asbestos filler in the prior flexibilized, cycloaliphatic epoxy resin compositions. However, asbestos had unique thickening properties difficult to duplicate, and some sag problems may still result when the feed tube liner or other coated circuit breaker article is heat cured at 130.degree. C. to 140.degree. C.
There has been a long felt need then, for sag resistant coating compositions that do not contain toxic materials and which are adequately arc and track resistant in the presence of moist, pressurized, arced sulfur hexafluoride gas.